In photolithography, a circuit pattern is formed by a reticle or photomask (hereinafter referred to simply as a "mask") on a substrate or wafer. The mask is typically a sheet of glass or similar material on which are formed patterns made from chromium or chrom-oxide. Stretched across the top of the mask is a pellicle which protects the mask from foreign matter such as dust and the like. The pattern of the mask is transferred onto the substrate during the photolithography process. However, if foreign matter such as large particles adhere to the pellicle, the image of the foreign matter will affect the pattern formed on the semiconductor wafer, resulting in a defective circuit pattern. Any defects in the circuit patterns in turn results in a reduction in manufacturing yield. Consequently, to assure defect free transfer of the pattern to the wafer prior to exposure of the pattern on the mask onto the wafer surface, the pellicle must be inspected to verify that foreign matter is not present.
FIG. 7 is a schematic perspective view from an oblique angle of the structure of a conventional foreign matter inspection apparatus as disclosed in U.S. Pat. No. 5,473,426. The apparatus shown in FIG. 7 emits from semiconductor laser 11 a radial beam of laser light of approximately 780 nm wavelength. The beam of laser light is transformed into a parallel beam by collimator lens 12. The beam is then expanded in the X direction in FIG. 7 by anamorphic prism 13 to produce a laser light beam with an elliptical cross section perpendicular to the X direction. Stop 14, which has a parallelogram-shaped aperture, then partially blocks the beam in the longitudinal or X direction. Mirror 15 then reflects the beam onto the surface to be inspected, pellicle 21, at an angle of incidence .theta. close to 90.degree. as measured with respect to the surface normal of pellicle 21.
The beam of light reflected onto pellicle 21 by mirror 15 forms a band-shaped illumination region 30 extending in the X direction across the pellicle 21. Band-shaped irradiation region 30 constitutes the region on pellicle 21 to be inspected. Foreign matter present in this band-shaped illumination region 30 scatters the light incident the region. The scattered light from the foreign matter passes through light receiving lens 31, and forms an image on linear image sensor 20. The intensity of the scattered light detected by linear image sensor 20 helps determine the size of the foreign matter. The system inspects the entire surface of the pellicle 21 spread across mask 22 for foreign matter by moving the mask 22 and pellicle 21 in a direction perpendicular to the direction of the band-shaped illumination region 30, the Y direction in FIG. 7, so that the band-shaped illumination region 30 moves across the entire surface of pellicle 21. Thus, the conventional inspection system depicted in FIG. 7 has one optical system which combines an incident light system with a receiving light system.
The trend in the semiconductor industry has been to use larger masks with much finer circuit detail. As a consequence, the reduction ratio of the projection lens in photolithography equipment is often increased from 4.times. to 8.times.. Thus, larger masks are needed to exposure the same area. This has increased the size of the area to be inspected while at the same time requiring inspection systems which are more sensitive. In the liquid crystal display industry, larger and preciser displays are required. To meet these needs conventional inspection systems, which must view a larger inspection region with the same or larger numerical aperture, have had to have the diameter of the light receiving lens and the size of other parts of the inspection system increased. The larger inspection systems (in particular the light receiving portion of the systems) also require a larger amount of space. The use of larger parts has increased the costs of these systems and has resulted in lower yields. The size of the incident light systems have also increased attendant with the increase in the size of the inspection region.
Additionally, use of one light receiving system in conventional inspection apparatus of increasingly larger size has also resulted in a significant difference in the sensitivity of detecting foreign matter between the center and the edges of the larger inspection regions. Thus, the system becomes much less sensitive to foreign matter towards the periphery of the inspection region viewed. This is in part due to the fact that the angle of the optical axis of the light receiving system with the region to be inspected varies greatly between the center and edges of the region being inspected.